Error when qt program is running: undefined reference to `_imp___ZN10QTcpSocketD1Ev'

出现的错误：
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN10QTcpSocketC1EP7QObject'
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN10QTcpSocketD1Ev'
　　undefined reference to `_imp___ZN15QAbstractSocket13connectToHostERK7QStringt6QFlagsIN9QIODevice12OpenModeFlagEE'
　　undefined reference to `_imp___ZN15QAbstractSocket5closeEv'

　　collect2: ld returned 1 exit status

解决方法：

Link error, check if
　　 QT += network is missing in .pro file